Two-phase flow liquid metal magnetohydrodynamic (LMMHD) systems have a number of significant potential advantages over conventional or other energy conversion systems.
First, the thermal efficiency of LMMHD systems is close to the Carnot cycle efficiency due to substantial reheat of the expanding gas by the coflowing liquid metal. In this way the expansion of the gas is almost isothermal.
Second, an LMMHD system has minimal or no moving parts. This makes such a system considerably less expensive to manufacture and maintain. In addition, absence of highly stressed high temperature moving parts (such as blades in thermal turbines) should permit increasing the top operating temperature without appreciable additional costs. In this way, thermal efficiencies could be raised by an additional ten percentage points or more. Combined with the first advantage this means that LMMHD systems could potentially produce 50 percent more electrical energy from the same heat source than conventional systems.
A third advantage is that LMMHD systems can operate over a large temperature range, i.e., from 300 to 3000K. This means that they are suitable for a variety of heat sources such as waste heat, cogeneration or conventional fuels as usual or at higher temperatures.
The fourth advantage is the very high electrical conductivity (10.sup.5 to 10.sup.6 mho/m).
The fifth advantage is that a wide range of power sizes are possible, from a few kW to several hundred MW.
Fabris, G. and Hantman, R. G., "Interaction of Fluid dynamics Phenomena and Generator Efficiency in Two-Phase Liquid-Metal Gas Magnetohydrodynamic Power Generators," Energy Conversion and Management an International Journal 21:49-60, 1981, and Pabris, G. and Pierson, E. S., "The Role of Interfacial Heat and Mechanical Energy Transfers in a Liquid-Metal MHD Generator," Energy Conversion an International Journal 19:101-118, 1979, have discussed the need to operate an LMMHD generator efficiently at a high void fraction in order to fully realize the potential advantages of LMMHD systems. In the past, the main cause of a decrease of the LMMHD generator efficiency was the slip loss (Fabris, G., "Formulation of the Slip Loss in a Two-Phase Liquid-Metal Magnetohydrodynamic Generator," Progress in Astronautics and Aeronautics 84:218-224, 1983) which occurred due to the transition of a two-phase flow pattern from bubbly to churn turbulent flow at higher void fractions. An improper flow pattern could be created by a poorly designed two-phase flow mixer.
Proper design of the mixer is discussed by Fabris, G., Kwack, E., Harstad, K., and Back, L. H., "Two-Phase Flow Bubbly Mixing for Liquid Metal Magnetohydrodynamic Energy Conversion," Proceedings of the 25th Intersociety Energy Conversion Engineering Conference, 2:486-493, Reno, Nev., 1990. It was shown that a properly designed mixer can create a low slip homogenous bubbly flow pattern at high void fractions. Another major cause of an improper flow pattern is coalescence of bubbles away from the mixer.
Fabris, G., "Discussions on Liquid Metal Magnetohydrodynamics," Proceedings of the 6th International Conference on Magnetohydrodynamics Electric Power Generation VI:427, Washington, D.C., 1975, was the first to suggest that the surface activity of liquid metals can be used to prevent coalescence of bubbles and therefore to maintain a bubbly flow pattern at much higher void fractions. In this way, very significant improvements in the performance of an LMMHD generator can be obtained over earlier experimental results.